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  <title>Document</title>
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<body>
  <script>
    // 1. 封装函数, 名字为sum
    // 2. 功能: 根据传入的两个数, 求和并且返回求和的结果（函数必须有return返回值）

    function sum(a, b) {
      let re = a + b
      return re
    }
    let res = sum(10, 20)
    console.log(res);
  </script>
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